Complementary Metal Oxide Semiconductor (CMOS) technology forms the basis of modern computing systems. The huge popularity of portable electronic devices such as smart phones and tablets means that computing technology is required which can function at moderate power while maintaining low leakage power. To meet this demand, modern CMOS technology uses low operating voltages and comprises transistors with a high operating threshold.
These features, however, mean that the CMOS will be sensitive to even small changes in the supply voltage. Sensing circuits which can detect changes in process temperature and voltage (PVT) and adjust certain parameters of a system accordingly provide one solution to the above problem. These sensing circuits cannot control for power variations due to changes in the clock cycle.
Due to the high power consumption of smart phones and tablets, modern CMOS technology employs aggressive clock gating to try and preserve battery life. When an electronic device is not being used, the clock signals will be turned off, and will only be turned on again when the system is operated by a user or due to internal commands. This aggressive clock gating requires large changes in the current that needs to be drawn from the supply. Load regulation of the power supply will cause the power supply to adapt to these changes in the current. In addition, package network inductance of the system means that any changes in current will induce a change in voltage of the system conductors. All these factors result in a momentary dip in the voltage at the CMOS transistors when clock gating occurs. This can be seen in the form of transients in the power supply when the electronic device is turned on, as the power supply adjusts to provide functional power levels for the electronic device.
In order for the electronic device to be fully operational, this voltage loss needs to be recovered as soon as possible. An analog solution is to provide additional voltage at the transistors to compensate for the voltage loss. This negates the benefits of low operation voltages of the CMOS transistors, however. A digital control method offers a more elegant solution.